1. Field of the Invention
This invention relates generally to electronic amplifiers. More particularly, the invention relates to a wide bandwidth radio frequency hybrid amplifier.
2. Description of the Related Art
Lowering distortion in solid state power amplifier circuits without compromising their transient response is an omnipresent problem. High frequency amplification is widely used in communications and broadcasting and also where high-speed switching is required for use in digital instrumentation. However, high frequency amplifier applications extend linear operation into areas where parasitic effects of interelectrode capacitance, wire inductance, stored charge and even operating frequency wavelength begin to adversely effect and dominate circuit behavior. Most amplifier topologies show a rolloff of gain with increasing signal frequency due to the effects of load and junction capacitance.
Load capacitance is not the only adverse effect when designing for high frequency operation. One important aspect that has serious impact on high speed and high frequency circuits is the existence of capacitance in the external circuit and in the transistor junctions themselves. The subtle effects of capacitance witnessed at low frequencies often dominate circuit behavior at high frequencies. This effect of increasing capacitance is known as the Miller effect.
To counteract the Miller effect, the active devices are operated in a mode known as cascode which increases bandwidth through a reduction in Miller capacitance and provides additional isolation between the source, the power supply, and the gain circuitry. While cascading is known to those skilled in the art of electronics, to ameliorate or eliminate the effects of the Miller capacitance, most designs such as those used in military communications, commercial wireless and CATV hybrids employ straight forward topologies or have been used in predominantly narrowband applications.
Familiar devices such as bipolar silicon transistors and silicon field effect transistors are still used to amplify radio frequencies. However, silicon technology is reaching its upper limits of acceptable low distortion operation. Gallium arsenide devices are noted for their hyper fast forward and soft reverse recovery characteristics with low stored charge. Another fact favoring gallium arsenide devices is that the device parameters are stable over a wide temperature range.
As an example, FIG. 1 shows a plot comparing the performance of gallium arsenide (GaAs) devices substituting for silicon (Si) devices in the same push-pull amplifier circuit. The composite triple beat distortion, dBc (dB reference to a carrier) levels per power output dBmV are improved using the GaAs devices. The use of GaAs technology for the active signal components maintains high gain while offering lower distortion figures and excellent linearity. However, mere substitution of a GaAs device into the same circuit topology optimized for bipolar silicon transistors or metal oxide semiconductor field effect transistors (MOSFETs) fails to fully capitalize on GaAs metal semiconductor field effect transistor (MESFET) potential. As discussed earlier, a standard hybrid amplifier arranged as a transformer-coupled cascode, balanced push-pull, using silicon technology typically incorporates paralleled cascoded devices to obtain higher output levels while maintaining low distortion figures. Prior art attempts to substitute GaAs devices base their designs on the same circuit topologies relying on traditional common-source common-gate cascodes. While achieving improved distortion compared to silicon technology, to meet the same system specifications, a compromise regarding performance, operating conditions or additional circuit complexity would be required.
A simple circuit topology for a wide bandwidth, ultra-linear amplifier with a frequency response extending to 1,000 MHz and beyond that exploits the performance of GaAs technology is desired.